A number of surgical procedures utilise electrosurgical or radiofrequency (RF) ablation techniques. Electrical current is usually delivered through a surgical instrument, or catheter, to the tissue requiring treatment. The return path of the electrical current is generally directed back through a large dispersive electrode. Burns associated with excessive heating at such a dispersive electrode complicate 0.1%-4% of procedures utilising RF ablation and can cause serious morbidity.
Monitoring the temperature of the ablation electrode improves the safety of the ablation procedure and can usually be achieved by inserting a single temperature sensor at the tip of the catheter. The need for improved temperature monitoring at the dispersive electrode has been increasingly recognised recently, for example, Canadian Patent No. 2,280,313 in the name of Vilos (1998) and Steinke et al ‘Dispersive pad site burns with modern radiofrequency ablation equipment’ (see Surgical Laparoscopy, Endoscopy and Percutaneous Techniques Volume 13 Issue 6 Pages 366-71). However, monitoring the temperature of the dispersive electrode is considerably more difficult due to its large size. Dispersive electrode burns are frequently caused in circumstances where most of the dispersive electrode is not in contact with the patient, which results in a high current flow in the remaining section of the electrode in contact with the patient. In an attempt to detect this situation, numerous (>20) temperature monitoring devices may have to be positioned throughout the dispersive electrode. However, measuring temperatures at so many points using conventional sensors, such as thermocouples or thermistors, is cumbersome and makes the dispersive electrode costly.
Because of the difficulty of measuring temperature at the dispersive electrode, several systems have been proposed attempting to detect partial failure of the dispersive electrode by monitoring the impedance during ablation. Systems that measure the impedance (e.g., U.S. Pat. No. 6,063,075 issued to Mihori on 16 May 2000, U.S. Pat. No. 4,848,335 issued to Manes on 18 Jul. 1989, and U.S. Pat. No. 4,494,541 issued to Archibald in 22 Jan. 1985) or the voltage and current (e.g., U.S. Pat. No. 5,817,091 issued to Nardella et al. on 6 Oct. 1998) fail to address all cases, because a dispersive electrode that has partially detached from the patient has a small surface area, but may still have good contact in the remaining, attached area, and hence a low impedance. This situation produces a high RF current density in the attached area, and hence has the potential to cause burns to the patient.
Some systems have been designed that attempt to measure the adequacy of dispersive electrode contact with the patient by monitoring the capacitance at the patient electrode (e.g. U.S. Pat. No. 4,303,073 issued to Archibald on 1 Dec. 1981) or measuring the impedance between two separate portions of the dispersive electrode (e.g., U.S. Pat. No. 4,416,277 issued to Newton et al. on 22 Nov. 1983). U.S. Pat. No. 6,258,085 issued to Eggleston on 10 Jul. 2001 discloses a system that monitors the impedance at two separate portions of the dispersive electrode and the quantity of thermal energy delivered to the patient, and calculates the probable amount of cooling at the return electrode to derive the probability of a patient burn. These systems have the disadvantage that they do not detect partial removal of the dispersive electrode if both parts of the return electrode are reduced to the same size (i.e. the pad dislodges parallel rather than perpendicular to its long axis). The temperature that is achieved at the dispersive electrode is determined by many factors that cannot be assessed accurately in all cases such as the local tissue perfusion, or degree of subcutaneous fat and fibrous tissue. Hence, a system that attempts to estimate the temperature exposes the patient to a higher risk than a system that directly measures the temperature. These systems also require their own specialised electrosurgical current generators and hence cannot be used in cases where another type of electrosurgical current generator is required (e.g., where a specialised radiofrequency current generator capable of measuring temperatures at multiple thermocouples is required to deliver current to a specialised cardiological catheter). However, cases have been reported of burns being produced despite the use of these devices in the medical procedures.
Thermochromic liquid crystals (TLC) and other thermochromic materials have been used in medical applications to monitor the temperature of skin.
M. Parsley, “The Use of Thermochromic Liquid Crystals in Research Applications, Thermal Mapping and Non-Destructive Testing,” Semiconductor Thermal Measurement and Management Symposium, 1991 SEMI-THERM VII, Seventh Annual IEEE Proceedings, 12-14 Feb. 1991, pp. 53-58 describes thermochromic liquid crystals.
U.S. Pat. No. 5,124,819 issued to Davis on 23 Jun. 1992 and entitled “Liquid Crystal Medical Device Having Distinguishing Means” describes a thermochromic device to detect skin temperature differences that may be associated with disease. A liquid crystal device has two layers of encapsulated thermochromic material for providing a color response with respect to temperature. The temperature ranges of color response of the two liquid crystal layers are different. A mechanism distinguishes the temperature range in which the device is operational. For example, a thermochromic strip may be fashioned into an elastic and deformable strip to monitor the temperature of a curved section of the body and detect disease states, such as cancer.
U.S. Pat. No. 4,649,923 issued to Hoffman on 17 Mar. 1987 and entitled “Temperature Indicating Electrotherapy Electrode” describes a temperature indicating electrotherapy electrode using liquid crystals. The temperature indicating electrotherapy electrode applies an electrical or electromagnetic signal to tissue of a living body and measures the physiological response of the tissue using the temperature responsive liquid crystal. The temperature indicating electrotherapy electrode is a small patch electrode that comprises a conductive metal foil used as a conductive patch electrode, the temperature responsive liquid crystal coated on the electrode, and a band of adhesive is provided around the outside of the electrode and liquid crystal layers. A color-temperature reference coating is provided as a border. A smaller sized electrode having this configuration is used relatively close to the treatment site with a larger dispersive return electrode used at a more remote location. In this manner, the arrangement ensures a higher density current at the treatment site and a lower, non-biologically stimulating signal at the larger electrode. The liquid crystal layer is sensitive to physiological temperature changes of 26° C. to 36° C., which includes the normal skin temperature range of 30° C. to 33° C. The small patch electrode may be used in therapies such as transcutaneous electrical nerve stimulation (TENS) in which electrical current is delivered to a patient through the small patch electrode and exits the patient through the larger dispersive electrode. The thermochromic layer is used to monitor the effectiveness of the delivered therapy. Such therapeutic electrodes are used to deliver very low intensity electrical energy (e.g., <0.1 watt) and cannot be used as a dispersive electrode during RF ablation. In RF ablation, high power (e.g., 20-300 watts) current is delivered. Using such an electrode in this application would lead to a high current density and subsequent skin burns due to the small electrode size. The low operating temperature range of these electrodes would result in the thermochromic layer changing colour in all cases when applied to the skin. This would make the electrodes unsuitable for detecting dangerous temperature elevations due to their lack of specificity.
Thus, a need clearly exists for a system of monitoring the temperature of a dispersive electrode at multiple sites during RF ablation to prevent burns being produced. The system must also be easy to monitor and able to detect dangerous temperature elevations with high sensitivity and specificity.